Abstract

Organic-rich Mississippian carbonates and oil samples from wells completed in the Mississippian and Woodford zones in northern Oklahoma were sampled and geochemically assessed to evaluate charge history. Rock and oil samples were collected from the Cherokee platform and the Anadarko shelf. Samples were analyzed using gas chromatography and gas chromatography-mass spectrometry (GC-MS) techniques for quantitative analysis of diamondoids and saturate and aromatic biomarkers. Results indicate Mississippian hydrocarbon source rocks have generation potential and reached the early oil window thermal maturity. Extracted bitumen from Mississippian rocks and related oils show unique biomarker signatures such as the presence of extended tricyclic terpanes and high input of C₂₇ relative to C₂₈ and C₂₉ in regular and rearranged steranes. The extent of cracking, as measured by diamondoids, reveals a dramatic change in diamondoids concentration between areas east and west, respectively, of the Nemaha uplift. The higher concentration of diamondoids and biomarkers observed west of the Nemaha uplift indicates mixing of uncracked oil with cracked oil migrating out of the Anadarko Basin. This mix of uncracked and cracked oils west of the uplift suggests episodic hydrocarbon charge and a long-distance component to the migration model. In contrast, the Mississippian samples from east of the Nemaha uplift are depleted in diamondoids, suggesting limited migration distance and localized hydrocarbon generation under lower thermal stress.